Man i'm realy worried about this fact. I saw that post "Click on my ads" http://forums.digitalpoint.com/showthread.php?t=614955 and it turned out that some one tried to ban some one else's adsense account. Theres another post http://forums.digitalpoint.com/showthread.php?t=612122 some one put 2 ads and asked for clicking on them. I'm sure those ads are not his own beside some one else's. That means if i got an enemy or some one tries a practical joke on me i could loose my adsense account. Google must do something about this seriously.
But there is something like "allowed sites", so you can prevent it. Ads are showed on 'the other' sites, but clicks are not counted.